The present invention relates to an apparatus for detecting synchronization (sync) words out of a received signal and, more particularly, to a sync word detection apparatus applicable to a mobile satellite communication system for detecting sync words out of a demodulated signal produced by demodulating a digital signal sequence in which a sync word having a predetermined length has been inserted by a transmitting station.
While a mobile satellite communication system offers a broad service area and a broad channel band, it suffers from fluctuations in the intensity of received field ascribable to multi-path fading and from so-called shadowing occurring when a mobile station gets behind a building or similar ostacle. Especially, shadowing causes the intensity of received field to sharply change and cannot be sufficiently compensated for even when an automatic gain control circuit is used.
In a system using voice activation method, for example, sync words indicative of the beginning and end of a voice signal are added to the leading and trailing ends of a digitized voice signal. The voice signal with such sync words are modulated and then transmitted to a receive station. The sync words allow the receive station to detect the beginning and end of the voice signal. However, when the receive station fails to detect the sync words due to shadowing or similar cause, it cannot reproduce the voice signal sequence at all.
A system of the type causing a transmit station to insert a sync word in data has a drawback that when a receive station fails to detect a sync word, data that follows the non-detected sync word are lost. Obviously, therefore, the detection of sync words is a prerequisite even with a mobile satellite communication system in detecting data sent from a transmit station.
In a mobile satellite communication system as well as other systems, it is a common practice to detect a sync word out of a signal received from a satellite by detecting a correlation. Specifically, the degree of coincidence of a demodulated signal undergone hard decision and a sync word is calculated, and it is determined that a sync word has been detected if the number of non-coincidences is smaller than a predetermined number (.epsilon.). For details of circuitry for so detecting a sync word, a reference may be made to William W Wu "Elements of Digital Satellite Communication", COMPUTER SCIENCE, 1984, pp. 342-348.
Misdetection probability P.sub.miss and false detection probability P.sub.f are the important criteria for the evaluation of a sync word detection system. Misdetection probability P.sub.miss is the probability that the system fails to detect a sync word despite the arrival thereof due to more than .epsilon. non-coincidences. The probability P.sub.miss is expressed as: ##EQU1## where p is the bit error rate on the transmission path, and N is the length of a sync word.
On the other hand, when only noise is received in a no-signal condition due to the decrease in the intensity of received field, the result of hard decision may happen to be less than .epsilon. non-coincidences. Then, the probability that the individual bits of the hard decision result on a demodulated signal coincide with the individual bits of a sync word is 1/2. Hence, false detection probability, P.sub.f, i.e., the probability that the system determines that a sync word exists is produced by: ##EQU2##
As the equations (1) and (2) indicate, to reduce the probabilities P.sub.miss and P.sub.f, the length N of a sync word has to be increased. Increasing the length N, however, would obstruct the efficient use of transmission path.
In the light of the above, there has been proposed a method which determines a correlation value between a demodulated signal undergone soft decision and a sync word pattern and detects a sync word by seeing if the correlation value is greater than a predetermined threshold value. This kind of method is disclosed in, for example, Japanese Patent Laid-Open Publication No. 38049/1987. A problem with this method is that when the demodulated signal subjected to soft decision diminishes due to a sharp fall of the received field intensity, the correlation value itself decreases despite the reception of a sync word. Once the correlation value decreases below the threshold value, the sync word cannot be detected. Since the fall of received field intensity due to shadowing or similar cause cannot be sufficiently compensated for even with an automatic gain control circuit, an acceptable countermeasure against the above problem is not attainable.